Gregory B. Raupp

Mössbauer investigation of supported Fe catalysts: III. In situ kinetics and spectroscopy during Fischer-Tropsch synthesis

In situ constant velocity Mossbauer data have been used to measure the rate of carbiding of 10FeSiO2 and 10FeMgO during the Fischer-Tropsch synthesis reaction at 523 °K and 3.3 H2CO. Simultaneous spectroscopic and kinetic measurements reveal the remarkable result that the reaction rate follows the extent of bulk carbide formation and, thus, that incorporation of carbon into the iron particles controls the number of active surface sites until carbiding is complete. Conversion, paraffin to olefin ratio, and C2C1 ratio increase with extent of carbiding. Hydrogenation of a fully carbided catalyst at 523 °K is slow and produces only methane.

Mössbauer investigation of supported Fe and FeNi catalysts: II. Carbides formed Fischer-Tropsch synthesis

Abstract Mossbauer characterization of iron catalysts following Fischer-Tropsch reaction at 523 °K reveals significant differences in carbide phases formed depending on particle, support, and alloying with Ni. Small iron particles on silica favor formation of the unstable ϵ′ and ϵ carbides. Large iron particles on silica and small particles on magnesia form χ carbide as is normal for bulk unpromoted iron. Alloying iron with nickel in approximately equimolar amounts completely prevents formation of bulk carbides at our reaction conditions, but some evidence exists for formation of surface carbon. The complexity of the metastable Fe/Fe x C/C system is further exemplified in a series of spectra recorded at intermediate reaction times. These data demonstrate time dependency of carbide phases formed and dependence of the rate of carbide formation on average metal particle size.

Polyimide-based intracortical neural implant with improved structural stiffness

A novel structure for chronically implantable cortical electrodes using polyimide bio-polymer was devised, which provides both flexibility for micro-motion compliance between brain tissues and the skull and at the brain/implant interface and stiffness for better surgical handling. A 5–10 µm thick silicon backbone layer was attached to the tip of the electrode to enhance the structural stiffness. This stiff segment was then followed by a 1 mm flexible segment without a silicon backbone layer. The fabricated implants have tri-shanks with five recording sites (20 µm × 20 µm) and two vias of 40 µm × 40 µm on each shank. In vitro cytotoxicity tests of prototype implants revealed no adverse toxic effects on cells. Bench test impedance values were assessed, resulting in an average impedance value of ~2 MΩ at 1 KHz. For a 5 µm thick silicon backbone electrode, the stiffness of polyimide-based electrodes was increased ten times over that of electrodes without the silicon backbone layer. Furthermore, polyimide-based electrodes with 5 µm and 10 µm thick silicon backbone layer penetrated pia of rat brain without buckling that has been observed in implants without silicon reinforcement.

Kinetics of the gas-solid heterogeneous photocatalytic oxidation of trichloroethylene by near UV illuminated titanium dioxide

Kinetics of the gas/solid heterogeneous photocatalytic oxidation of dilute trichloroethylene (TCE) vapors by ultraviolet-illuminated titanium dioxide have been determined using a fixed-bed dynamic photoreactor. Reaction rate dependences on inlet TCE, oxygen and water vapor concentrations were found to consist of both reactant sensitive and insensitive regions. In the reactant sensitive regions, measured limiting apparent reaction rate orders for TCE, oxygen and water vapor are 0.8, 1.7 and — 3, respectively. Water vapor in the reactant stream lowersinitial reaction rates relative to corresponding water free conditions, but is required to sustain photocatalytic activity for extended periods of time.

Mössbauer investigation of supported Fe and FeNi catalysts: I. Effect of pretreatment on particle size

Abstract Silica-supported Fe and FeNi catalysts show strong dependence of metal particle size on pretreatment, particularly the initial dehydration step. Room temperature Mossbauer spectra and supporting X-ray diffraction data reveal that mild pretreatment in the form of slow vacuum drying prior to reduction produces significantly smaller particles of Fe and FeNi on silica than either direct reduction or calcining. Once the catalyst precursor is completely dried, metal particles are relatively stable against further growth, suggesting that residual water or anions from the incipient wetness impregnation play a critical role in metal agglomeration during pretreatment. For 5 wt% Fe 5 wt% Ni on silica, vacuum drying and subsequent reduction produce FeNi alloy particles small enough to exhibit superparamagnetic behavior. Precalcining this catalyst increases the alloy particle size considerably as shown by a ferromagnetically split Mossbauer spectrum. Computer fitting of the broad peaks reveals significant amounts of metallic iron or extremely iron-rich alloy and, therefore, that phase separation can accompany particle growth in FeNi alloy catalysts.

Photocatalytic oxidation of oxygenated air toxics

Photocatalytic oxidation of dilute oxygenated organic compounds in air streams can readily be achieved at ambient conditions over near-ultraviolet (UV) illuminated titanium dioxide. Oxidation rates of acetone and methyl-t-butyl ether (MTBE) as a function of organic concentration exhibit limiting apparent reaction orders of 1.3 and 0.5, respectively. The dependence of oxidation rate on O2 concentration is approximately first order at low oxygen concentrations, with a saturation in the rate above 15 mol% O2. At low concentrations in the feed, water vapor has little effect on the reaction rate, but significantly degrades the rate at higher concentrations. In the absence of water vapor in the feed, high initial oxidation rates cannot be sustained and the catalyst eventually becomes completely de-activated. Photocatalytic activity can be maintained for extended time on stream by including water vapor in the reactor feed. Oxidation rates are strong functions of UV intensity. We have measured quantum yields greater than unity, suggesting that oxidation occurs through a complex surface-mediated, free radical chain reaction mechanism.

Infrared spectroscopic investigation of gas-solid heterogeneous photocatalytic oxidation of trichloroethylene

Abstract Transmission infrared spectra of untreated titania reveal that the surface is highly hydrated and contains both hydroxyl groups and chemisorbed water, and that in this state the surface does not chemisorb trichloroethylene (TCE). Near-ultraviolet (UV) illumination in the presence of TCE vapor leads to desorption of molecular water and subsequent formation of several adsorbed hydrocarbon intermediates and carbon dioxide. Formation of bands at 1609 and 1302 cm −1 suggests the presence of an adsorbed dichlorinated olefin. An intense band at 1743 cm − is assigned to the symmetric CO stretch of dichloroacetaldehyde. Comparison of the OH stretching regions of the spectra collected during illumination in the absence of TCE to those collected during illumination shows that hydroxyls are consumed in the TCE oxidation reaction. These observations are consistent with a mechanism in which water desorption is a prerequisite “trigger” step for oxygen adsorption and subsequent reactive hydroxyl radical and hydroperoxide radical formation, as well as for TCE adsorption. The evidence suggests that attack of adsorbed olefinic derivatives of TCE by hydroxyl radicals or by hydroperoxide radicals leads to production of the aldehydic intermediate. Ultraviolet illumination stimulates desorption of product CO 2 molecules formed by further attack of the aldehyde by reactive radicals and surface decomposition of the resulting intermediates.

Effect of varying titania surface coverage on the chemisorptive behavior of nickel

Abstract Temperature-programmed desorption (TPD) of CO and H2 from nickel surfaces containing varying amounts of titania showed that the effects of titania adspecies are predominantly short-ranged. Titania surface species block CO adsorption at strongly bound sites (believed to be sites atop individual Ni atoms) corresponding to heats of adsorption in the range 134–139 kJ · mol−1; in addition, titania enhances adsorption into more weakly bound sites (believed to be bridging and hollow sites between Ni atoms) with heats of adsorption from 91 to 107 kJ · mol−1. For hydrogen adsorption, overall adsorption strength was increased with increasing concentration of surface titania. In addition, a new, activated adsorption state was created and is believed to be associated with hydrogen at sites on titania. The variations of the initial sticking coefficients of CO and H2 as a function of titania precoverages show that at low titania coverage each Tiox moiety influences approximately 4–10 Ni surface atoms. The long-range electronic influence of titania at a concentration of 0.1 monolayer was estimated to be responsible for only a 6-kJ · mol-1 decrease in the CO heat of adsorption. Qualitatively, similar results were found for an alumina-containing Ni surface, although alumina appears to be more poorly dispersed than titania on Ni. This suggests that the effects of metal oxide species on metal surfaces may be generalized to include irreducible support materials.

First-principles modeling, scaling laws and design of structured photocatalytic oxidation reactors for air purification

Abstract First-principles, predictive engineering models provide a sound theoretical basis for quantifying the inherent light energy utilization capabilities and performance limitations of candidate commercial photocatalytic oxidation reactor configurations. In particular, these models provide insight into the similarities and differences between photoreactors based on structured honeycombed monoliths, and those based on reticulated foams or other random catalyst supports. For honeycombed monoliths, a deterministic first-principles radiation field model provides the channel wall light intensity profile down the length of a single channel in the monolith. A three-dimensional developing flow convection–diffusion reaction model employing this radiation field submodel predicts the velocity and concentration fields. The model shows that light intensity gradients in a monolith of typical dimensions are severe, that only a fraction of the monolith can be effectively photo-activated, and as a consequence process performance is largely controlled by light distribution. For a given light source and photocatalyst combination, reactor performance scales according to the aspect ratio of the channeled monolith, the Reynolds number, and the Dahmkohler number. For randomly structured monoliths, the radiation field must be determined by probablistic methods. Monte Carlo simulations show that the radiation field in such random porous structure scales according to the pore size distribution and the void fraction, and the photocatalyst film thickness. Reactor performance scales by the radiation field, the Peclet number, the Stanton number, and the Dahmkohler number. The complex interrelationship between the random structure of the monolith and the resulting radiation field and mass transfer behavior makes scaling of these reactor types particularly difficult.

The role of oxygen excitation and loss in plasma‐enhanced deposition of silicon dioxide from tetraethylorthosilicate

The deposition rate of silicon dioxide from tetraethylorthosilicate/O2 capacitively coupled plasmas increases with increasing applied rf power, increasing total pressure and decreasing wafer temperature. These measured deposition rate dependences can be explained by a simple plasma deposition model in which deposition occurs through both an ion‐assisted and an oxygen atom initiated pathway. The relative contributions of these pathways were roughly isolated using limiting step coverage measurements on low aspect ratio trenches. Limiting step coverages decreased, and hence directionality increased, with increasing rf power density, decreasing total pressure, and increasing wafer temperature. A simple bulk plasma chemistry model combined with an analytical sheath model was developed to qualitatively explain our experimental findings. The model suggests that the ion‐enhanced deposition rate is directly proportional to oxygen ion flux, with a reactive sticking coefficient approaching unity. Using literature va...